1. Field of the Invention
The present invention is directed to a method of producing cyclic carbonic acid esters. More particularly, the present invention involves a low cost method of producing trimethylene carbonate.
2. Description of the Prior Art
Cyclic carbonic acid esters are used, for example, as building blocks of potentially biodegradable polymers. A particular cyclic carbonic acid ester, trimethylene carbonate (1,3-dioxan-2-one) may be used in a variety of applications, such as for surgical stitching material, vessel implants and apparatus for osteo-synthesis. Trimethylene carbonate is a desirable monomer to use because of its unique property of not decreasing in volume on polymerization.
Trimethylene carbonate may be used as a monomer in the synthesis of poly(trimethylene carbonate) polyols, which are used in flexibilizing acrylic melamine coatings. Trimethylene carbonate may also be used to make surgical sutures and modified polyurethane elastomers. Poly(trimethylene carbonate) polyols improve both ambient and low temperature flexibility and reduced the viscosity of urethane coatings formulated with selected commercial acrylic polyols.
For industrial production of trimethylene carbonate, it is desirable to find a method of synthesis yielding cyclic carbonates that can be produced in high yields, by a relatively simple industrial process. Numerous methods are known for producing carbonic acid esters such as trimethylene carbonate. For example, the trans-esterfication of diethylcarbonate with 1,3-propanediol in the presence of sodium or sodium methoxide to obtain trimethylene carbonate is one of the oldest methods of production (W. H. Carothers and F. V. Natta, J Am. Chem. Soc. 52 (1930) 322), but the low yield makes this method unattractive for industrial use.
U.S. Pat. No. 5,212,321 to Muller et al. discloses a method for producing trimethylene carbonate wherein 1,3-propanediol is reacted with diethylcarbonate in the presence of zinc powder, zinc oxide, tin powder, tin halide or an organo-tin compound at an elevated temperature. However, the Muller et al. process is very expensive as the process the separation, isolation and disposal of residues and catalysts, which are time consuming and expensive.
U.S. Pat. No. 5,091,543 to Grey discloses a method of preparing five- and six-membered cyclic carbonates. The method involves reacting a 1,2- or 1,3-diol with an acyclic diester of carbonic acid in the presence of a catalyst selected from alkylammonium salts, tertiary amines, and ion-exchange resins containing alkylammonium or tertiary amino groups. Cyclic carbonates free of carbonate based product by-products are obtained. However, the Grey process is also very expensive as the process requires the use of reactors made from materials of construction that will not corrode when exposed to the halide ions in the process. Isolation and disposal of residues and catalysts are also time consuming and expensive.
Another process used to prepare trimethylene carbonate involves reacting 1,3-propanediol with urea in the presence of zinc based catalysts. This type of process is described, for example, in Japanese Patent Nos. 7-330686 and 7-330756. The process requires expensive and time consuming isolation, recovery and recycling of the catalyst.
Trimethylene carbonate has also been made by reacting 1,3-propanediol with ethylchloroformate in the presence of two equivalents of triethylamine using tetrahydrofuran as a solvent (Toshiro Agriga et al., Macromolecules 30 (1997) 737). However this method produces trimethylene carbonate in low yield, and requires expensive and time-consuming recovery of the amine and the solvent.
Linear carbonate polymers can be prepared from carbonic dihalides and diols using catalytic amounts of nitrogen containing bases. U.S. Pat. No. 4,365,055 to Madigan, for example, discloses a method of preparing a carbonate based product by introducing phosgene to an anhydrous solution of at least one substituted or unsubstituted 1,3-propanediol in a solvent. A particular teaching of the Madigan patent is that coproduction of cyclic carbonates is reduced by the presence in the solution of a catalytic amount of a nitrogen-containing base such as pyridine. The method disclosed by Madigan et al. requires the use of catalytic quantities of nitrogen containing, hydrohalide salt-forming bases in order to produce linear carbonate polymers. The method according to Madigan is also expensive because it requires solvent removal.
There remains a need for a low cost method for producing trimethylene carbonate. A low cost method desirably involves production of trimethylene carbonate in relatively high yields. Further reductions in cost could be attained if trimethylene carbonate could be made without requiring use of catalysts or acid scavengers with their associated expenses for clean up and/or recycling or disposing of residues of catalyst material or hydrohalide salts. A combination of several or all of the desirable features would be even more desirable.
The present invention is directed to a method of synthesizing trimethylene carbonate comprising the steps of:
reacting 1,3-propanediol and either phosgene or a bis-chloroformate, without an HCl scavenger, to form a polycarbonate based intermediate having the 
xe2x80x83where n is at least 2, R1 is an end group and can be H, R3xe2x80x94Cl, R3xe2x80x94H or xe2x80x94C(O)Cl, where R3 is C1-C6 linear or branched alkyl and R2 is an end group and can be xe2x80x94OCH2CH2CH2Cl, xe2x80x94OCH2CH2CH2OH or Cl; and
applying a combination of temperature and pressure at which trimethylene carbonate will be in the vapor phase, generating trimethylene carbonate vapors.
The present invention advantageously provides a method for making trimethylene carbonate with efficient high yields. Some embodiments of the invention provide the additional advantage of not requiring use of catalysts or acid scavengers. Although not required, the use of catalysts or acid scavengers is not precluded in other embodiments of the method of the present invention.